Training module

Created by Ali Zandieh, MD, MPH
NIH StrokeNet Fellow – University of Pennsylvania

This training module is designed to introduce the basic concepts involved in detection of microemboli using Transcranial Doppler (TCD). A basic familiarity with TCD is assumed. If you are unfamiliar with TCD, a TCD simulator teaching tool developed by one of the originators of TCD technology is available online and may be useful (http://www.transcranial.com/edu/index.html).

Let’s first watch some normal TCD recordings without any microembolic signals (MES).

Herein, we want to briefly explain different types of microemboli with special focus on their clinical significance. Cerebral microemboli can be detected in various medical conditions such as atherosclerotic carotid disease [1, 2], cardiovascular diseases [3, 4], and cardiovascular invasive interventions and surgeries [5, 6, 7]. According to their composition, emboli can be divided into two main subtypes i.e. solid and gaseous emboli [8, 9]. Particles of thrombus and atheroma originating from vascular or cardiac sources are examples of solid emboli and generally considered as the main cause of cerebral ischemic events [10, 11]. In contrast, in patients with mechanical prosthetic cardiac valves, gaseous microemboli are more common [12, 13]. Cavitation, which refers to the formation of bubbles from gas dissolved in the blood, is though to be responsible for development of gaseous microemboli [14, 15, 16].

For successful detection of microemboli signals, both spectrogram and M-mode should be closely examined.

When a microembolus passes through the insonated artery, a sharp “chirp” like sound is heard as the ultrasound energy reflects off the embolic material. This appears visually as a vertical linear bright signal within the Doppler spectrogram. When M-mode recording is available, passage of the microemboli from deep to shallow depths (as the emboli moves distally through the vessel) may appear as a linear slightly slanted streak across multiple depths and produces diagonal signal in M-mode. This reflects the movement of the emboli in relation to the Doppler probe over time. (Figures 1 and 2).

On the contrary, the artifacts are vertical and so can be differentiated from microemboli (Figure 3).

Below is an MES. Listen carefully and try to familiarize your ears with the chirping sound it causes. This sound can be quite obvious with large microemboli, but can be much more subtle with smaller emboli.

Further, a microembolus produces a very soft sound but the sound of artifacts is coarse and much louder. Listen carefully to these audios and try to differentiate the microembolus from artifact just by their sounds.

The other issue that can help in the differentiation of microemboli signals from artifacts is that microemboli are limited to the borders of the arteries. Thus, no one expects that the microemboli signals extend to the areas outside the signals of the arteries (Figure 3). By contrast, in both spectrogram and M-mode the artifacts go beyond the borders of artery (Figures 4 and 5).

Try to find the microembolus in the video.

As you saw, there is one microembolus in the video. As, it passes through the artery, you can hear a sound in your headphone and also see the embolus in the M-mode. Now, let’s look at the same video for one more time. In this video we have indicated the embolus with a pointer.